(a) Field of the Invention
The present invention concerns processes for the production of an atmosphere for the thermal treatment of metals and, more particularly, "of protection" atmosphere, such as used during annealing, tempering, heating before hardening, decarburizing annealing, brazing or sintering, comprising mainly nitrogen and controlled amounts of reducing, or carburizing and oxidizing, or decarburizing species.
(b) Description of Prior Art
In this type of thermal treatments, it is necessary to protect metallic parts against oxidation and, eventually, against decarburization by utilizing the protective atmosphere which is present in the furnace.
These protective atmospheres are frequently obtained from an exothermic generator to ensure an incomplete combustion of a hydrocarbon with air. Generally, the exothermic gas should be purified in order to limit the level of CO.sub.2 and reduce the dew point. The thermal treatment atmosphere may originate from an endothermic generator where a mixture of air and hydrocarbon is sent over a catalytic bed at a temperature higher than 1000.degree. C. In order to prevent a high proportion of carburizing species, the endothermic gas is diluted with pure nitrogen produced by cryogenic means.
It has also been proposed to produce thermal treatment atmospheres by reaction of a mixture of hydrocarbons and impure nitrogen having a residual content of oxygen higher than 0.5% at a temperature higher than 950.degree. C., with a nickel base catalyst. This process has the disadvantages of a significant energy cost by reason of the temperature required for the catalytic reaction and the formation of soots for which nickel is a promoter.